Space-time-power scheduling for wireless networks

ABSTRACT

A technique is disclosed to schedule frame transmissions in a wireless local area network. The network includes a plurality of stations configured to communicate on the same frequency channel with a plurality of access points. A central controller examines the transmission characteristics between the various stations and access points and identifies frames that may be simultaneously transmitted by a subset of the access points to their intended stations.

RELATED APPLICATIONS

This application is a Divisional of co-pending U.S. patent applicationSer. No. 11/404,309 filed Apr. 14, 2006, which is a Continuation of U.S.Pat. No. 7,031,336 issued Apr. 18, 2006, which claimed the benefit ofU.S. Provisional Application No. 60/406,165 filed Aug. 26, 2002, theentire contents of all of which is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to wireless networks, and moreparticularly, to space/time/power RF routing for such networks.

2. Related Art

Wireless local networks (WLANs) based on the IEEE 802.11 standard haveproven to be popular. IEEE 802.11 is a wireless standard related to theIEEE 802.3 standard established for wired Ethernets. In contrast towired networks, an IEEE 802.11 WLAN must conserve the limited bandwidthpresented by a wireless transmission medium. Accordingly, a set of rulesin the IEEE 802.11 standard is dedicated to medium access control (MAC),which governs accessing the wireless medium and sending data through it.

In an IEEE 802.11 WLAN, multiple wireless nodes or stations (STAs) suchas laptops may communicate through an access point (AP) with other usersof a wired local area network (LAN). To avoid medium access contentionamong STAs and APs, the MAC specifies a carrier sense multipleaccess/collision avoidance scheme (CSMA/CA) as controlled by adistribution coordination function (DCF).

Similar to Ethernet operation, the DCF first checks that the radio linkis clear before transmitting. If another transmission is detected, theDCF specifies a random backoff period (DCF inter-frame space or DIFS).Only if the channel is clear after the expiration of the random timeperiod, may a STA begin transmission. Because two STAs may communicatewith a given AP but be out of range with respect to each other, the DCFalso specifies a virtual carrier sense (VCS) procedure by which thewireless medium is reserved for a specified period of time for animpending transmission. For example, the VCS may be implemented by theuse of request-to send (RTS) and clear-to-send (CTS) frames. A given STAwould signal the beginning of a transmission with an RTS which specifiesa reservation period. Although other STAs may be out of range from thisRTS, the AP responds with a CTS that will be received by the remainingSTAs. This CTS will communicate the reservation period to all the STAs,which update their Network Allocation Vector (NAV) accordingly. The NAVacts as a timer for the reservation period and protects datatransmissions by causing the medium to be non-idle over the duration ofthe entire frame exchange.

While DCF with CSMA/CA reduces the possibility of collision, it does noteliminate it due to “hidden stations” reality. Further, DCF is notsuited for time-sensitive applications because of the unbounded delaysin the presence of congestion or interference. DCF may alsounder-utilize available bandwidth due to access contention. Thisinefficiency is increased because of the excessive overhead and the lowdata rate at which the preamble and PLCP header are transmitted. Whenthis is used with RTS/CTS (Request to Send/Clear to Send) controlframes, e.g., to handle the “hidden” station problem, inefficiency isincreased even further.

If greater bandwidth utilization is desired, another type of mediumaccess control called the Point Coordination Function (PCF) may beimplemented. PCF specifies the use of special stations in APs denoted aspoint coordinators, which act to ensure contention-free (CF) service.PCF is a centrally based access control mechanism (as opposed to adistributed architecture) based on a polling and response protocol,where DCF is based on CSMA/CA. During a contention free period (CFP), aSTA can only transmit after being polled by the Point Coordinator (PC).The PC may send polling frames to STAs that have requestedcontention-free services for their uplink traffic. If a polled STA hasuplink traffic to send, it may transmit one frame for each polling framereceived. The PC expects a resonse within a short inter-frame space(SIFS), which is shorter than a priority inter-frame space (PIFS).

However, PCF also has limitations, especially in WLANs with largenumbers of STAs and APs, e.g., in an enterprise application. Withtoday's needs, it is desirable to expand the coverage areas for 802.11WLANs, e.g., networks such as offices are expanding in size. However,current 802.11 networks utilizing DCF and PCF are limited in theirscalability to these larger networks, e.g., enterprise, as will bediscussed below. Enterprise, as used herein, refers to networks havingnumerous APs and STAs in large physical areas.

One limitation of PCF is that since the CFP repetition rate is notdynamically variable, there is a trade-off between low latencyapplications requiring a fast repetition rate and an efficient use ofthe medium requiring slower repetition rate. Also, the start of a CFperiod is not exactly periodic (i.e., it can only begin when the mediumis sensed to be idle). As a result, the CFP may be forced to end beforeserving some STAs on the polling list. Further limitations may be thatall the CF-pollable APs have the same level of priority since they aresimply polled by ascending Association ID (AID), and the PC has to pollall the STAs on its polling list even if there is no traffic to be sent.

In addition to governing medium access, DCF also specifies rules for thefrequency scheme that may be implemented in a WLAN having multiple APs.For example, FIG. 1 a shows a WLAN having coverage areas or cells 10corresponding to four APs. To minimize contention between adjacent APs,each AP is assigned a frequency channel for its cell 10. FIG. 1 a showsthat the overlap of cells 10 allows complete coverage within the WLAN.

IEEE 802.11 WLAN suffers from the rigidity required by the DCF. Inaddition, FCC regulations limit the amount of power that can be used bythe APs and STAs. The DCF rigidity and power allocation limits presentsevere challenges to users during network deployment and modification.Even if careful network planning is implemented, there may still be lossof bandwidth due to unpredictable circumstances such as a subscriber'smovement and activity level. In addition, the overall network bandwidthis limited by non-adjacent cell interference such that cells cannot bereadily reduced in size to boost bandwidth. Moreover, because frequencychannels are used for interference-avoidance planning, cell bandwidth islimited to a single AP bandwidth. When external interference is present,system frequency cannot be easily changed since the frequency resourcealready been used for another purpose.

Network bandwidth cannot be easily allocated to areas of high demand(such as conference rooms). Finally, as with any segregated medium, theDCF rigidity leads to a loss of trunking efficiency.

For example, FIG. 1 a illustrates an ideal situation where each cell 10has a circular coverage area. However, in reality, the coverage area ofeach cell 10 is not a circle. For example, in an enterprise application,such as in a building with large numbers of walls and offices, numerousAPs and STAs are needed to allow STAs to transfer information betweeneach other. The walls and other barriers result in non-uniform coverageareas for each cell 10.

FIG. 1 b shows coverage areas or cells 20 in a practical WLANenvironment. As seen, the coverage areas are no longer uniform circles,but are irregular having areas of broader coverage (the peaks) and areasof lower coverage (the nulls). For example, long peaks 25 may correspondto long hallways in the building. Because cells 20 do not have uniformcoverage, “holes” 30 exist in the network, where communication is notpossible. Holes 30 do not necessarily represent areas where no framesare able to be sent and received, but rather only a small percentage ofdropped frames are all that may be tolerated due to TCP/IP behavior,thereby effectively ending communication ability within that area.

A possible solution to “fill” holes 30 may be to increase the density ofthe APs in the WLAN, i.e., move the APs closer to each other, whichrequires more APs for the same outer coverage area. However, increasingthe density of the APs will result in increased interference between APsand STAs, while also increasing the cost of the system. Consequently, inorder to reduce interference, the transmit power of the APs must bereduced. But, this may again result in holes in the WLAN coverage due toirregular coverage “footprints” of the APs at an additional cost of areduction in maximum throughput of the system.

Accordingly, there is a need in the art for improved techniques toenhance cell coverage, eliminate the need for cell-based frequencyplanning, and increase network effective bandwidth and trunkingefficiency in WLANs, especially in enterprise.

BRIEF SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a wireless local areanetwork (WLAN) includes a plurality of M access point (AP) transceivers,wherein each access point transceiver controls its own multi-elementantenna. The network includes a plurality of N stations (STAs), whereineach station can communicate with all or a subset of the M access pointtransceivers. In addition, a central controller or radio frequency (RF)router is coupled to the plurality of M access points and is configuredto examine the “transmission characteristics” between a subset of thestations and a subset of the antennas for each access point.Transmission characteristics include path losses between transmitter andreceiver, transmission power, receive signal level, ratio between onetransmission path loss and another, and other types of suitableinformation metrics. Based upon these transmission characteristics, thecontroller identifies frames that may be scheduled for simultaneoustransmission from a subset of the access points to a subset of thestations, such that a given access point in the subset is uniquelyassigned a given frame for simultaneous transmission to its intendedstation with high probability of delivery.

In one embodiment, transmission characteristics include the relativepath loss between APs to STAs and between APs to APs and the probabilityof interference from STA to another STA (estimated from the relativepath loss). The relative path loss characteristics are also knowncollectively as a spatial signature. The transmission characteristicsare based on signals at the receiving STA and/or AP. Thus, when thecontroller has the transmission characteristics of the APs and STAswithin the network, it can select the best AP with which to transmit theframe to the appropriate STA. Selection metric is based on best signaldelivery path and minimum interference to co-existing transmissions. Ifframe transmission, regardless of the AP selected, would interfere withan existing transmission, the controller may decide to hold the frame afixed period of time until conditions change or drop the frame entirely.

A plurality of such networks can be utilized to create a larger system,with each network operating on its own unique frequency channel.

By providing the controller with the transmission characteristicsinformation and allowing the controller to select from multiple APs totransmit frames to multiple APs on a single frequency channel, systemperformance can be significantly improved. For example, the probabilityof having an outage is greatly reduced, the range can be extended (fromhaving the ability to select one of many APs instead of relying on justone AP), bandwidth is increased, interference is reduced, and systemcost is lowered.

The present invention exploits the natural large path-loss variabilitywithin indoor environments. This variability, for example, is a resultof extensive shadowing caused by walls and other objects and signalpropagation through hallways and air ducts. In addition, extensivesignal multipath propagation cause Rayleigh fading that furthercontributes to signal level variations.

The invention will be more fully understood upon consideration of thefollowing detailed description, taken together with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a illustrates cells with theoretical footprints for aconventional WLAN.

FIG. 1 b illustrates an example of cells with realistic footprints in anenterprise.

FIG. 2 is a block diagram of a WLAN according to one embodiment of theinvention.

FIG. 3 is an illustration of a channel gain matrix according to oneembodiment of the invention.

FIG. 4 is an illustration of the coverage areas for three access pointsindicating regions of relatively low station-to-station spatialsignature correlation and regions of relatively high station-to-stationspatial signature correlation.

FIG. 5 a is a graph of the cross-correlation vs. channel-to-interferenceratio (C/I) for a plurality of stations to demonstrate the subset ofstations that may be serviced simultaneously according to across-correlation threshold of 0.9.

FIG. 5 b is a graph of the cross-correlation vs. C/I for a plurality ofstations to demonstrate the subset of stations that may be servicedsimultaneously according to a cross-correlation threshold of 0.95.

FIG. 6 is an illustration of a plurality of networks according to oneembodiment of the invention.

FIG. 7 is a block diagram for an access point module according to oneembodiment of the invention.

FIG. 8 is an antenna gain plot for the antenna array used in the accesspoint module of FIG. 7.

FIG. 9 is a block diagram for an access point module in which the mediumaccess control (MAC) functions are implemented by the central controlleraccording to one embodiment of the invention.

FIG. 10 is block diagram for a central controller for the access pointmodule of FIG. 7.

FIG. 11 is a block diagram for a central controller for the access pointmodule of FIG. 9.

FIG. 12 is a state machine diagram for a probing process according toone embodiment of the invention.

Use of the same or similar reference numbers in different figuresindicates same or like elements.

DETAILED DESCRIPTION

FIG. 2 shows a block diagram of an exemplary IEEE 802.11 WLAN 15configured to implement the space-time-power (STP) frame schedulingtechnique of the present invention. A central controller 20 controls themedium access by a plurality of M access points (APs) (for illustrationclarity, only two APs 25 and 30 are shown in FIG. 2). Controller 20 mayalso be referred to as a router, an RF router, or radio processor. EachAP may communicate with a plurality of N stations (STAs) (again, forillustration clarity, only two STAs 35 and 40 are shown in FIG. 2). STAsmay include laptop PCs and handheld devices, such as PDAs. These devicescan be mobile, portable, or stationary. AP devices also contain 802.11conformant MAC and PHY interface to the wireless medium and provideaccess to a distribution system for associated stations.

Each AP 25 and 30, in one embodiment, may transmit on the same frequencychannel. The combination of APs 25 and 30 and STAs 35 and 40 form abasic service set (BSS), which is a group of stations that communicatewith each other on the same frequency channel through a group of accesspoints, according to one embodiment. In another embodiment, each APcreates its own BSS so that each AP has a unique BSSID. Although a STAis associated with a specific AP, multiple APs can feed a single STA bysetting the BSSID values to the same as that of the AP with which theSTA is associated with.

As described above, a conventional IEEE 802.11 AP will refrain fromtransmission based on normal DCF access control using NAV condition andpower detection. However, in WLAN 15, central controller 20 controlstransmission by APs 25 and 30 based upon the predetermined probabilitythat a given transmission will be successful (AP to STA and STA to AP).For example, this predetermined probability may be based upon aprediction of the signal-to-interference or channel-to-interference(C/I) ratio at the intended STA.

The variability in RF path loss between APs 25 and 30 may be increasedby having each AP 25 and 30 use a multi-element array. In turn, theenhanced variability in RF path loss increases the likelihood thatcentral controller 20 may schedule simultaneous frame transmissionswithout interference. The RF path loss variability is enhanced by theinherent large variance in typical WLAN environments (such as within anindoor office) due to shadowing and Rayleigh fading.

Central controller 20 executes a scheduling algorithm that exploitsknowledge of transmission characteristics between each STA 35 and 40 andthe multi-antenna elements of each AP 25 and 30 to maximize the numberof STAs that can be simultaneously served and the amount of datatransferred at each given time slot. To allow for simultaneoustransmissions in a single frequency channel, each STA 35 and 40 shouldbe well illuminated by a given AP 25 or 30. In addition, theillumination from interfering APs should be minimized.

To achieve these goals, central controller 20 may form a channel gainmatrix, which may also be denoted as the spatial signature matrix. Anexample of a channel gain matrix 50 for a WLAN having multiple APs andSTAs is shown in FIG. 3. Channel gain matrix 50 includes all channelgains/signal strengths between each AP antenna element/beam and each STA(in the AP/STA section) and the channel gains between each AP antennaelement/beam and neighboring AP antenna elements/beams (in the AP/APsection). Central controller 20 may use the channel gain matrix toschedule the transmissions by APs 35 and 40. Because of the relationshipbetween a received signal strength and the corresponding channel gain,channel gain matrix 50 may also be denoted as a signal strength matrix.

Central controller 20 may obtain its channel gain matrix by collectingsignal level values as received by APs 25 and 30. Generally, thesesignal values can be obtained at the corresponding APs by listening toeach STA on the transmission. If a specific STA has not been active foran extended period, central controller 20 may stimulate STA transmissionby forcing an AP to issue an RTS (Request to Send or any other frameinducing ACK transmission by STA) (in normal DCF mode) or by polling (inPCF mode). This forced transmission is denoted as probing and will beexplained further herein. From time to time, central controller 20 maydirect a given AP to stay in listening mode during the downlink periodso it can estimate its channel gain relative to the remaining APs toupdate the AP/AP section of the channel gain matrix.

The frame preamble transmitted by each STA and AP can simplify thecollection of signal strength values and signal-to-noise ratio orchannel-to-interference ratio (C/I) estimation. Because the structure ofa preamble is well known to each receiver, a matched filter can be usedto accurately determine the strength and timing of the incomingtransmission. If the preamble was received on multiple APs (or antennabeams in separate APs), central controller 20 may require that the framewill be demodulated by a single AP. By correlating the time of thedemodulated frame with the time of the measured preambles, centralcontroller 20 can associate the “preamble signature” with a specificSTA. Because a matched filter may be used for signal strengthestimation, the preambles may be measured even in the presence of othersignals arriving at the APs. (Cross correlation may be used in place ofa matched filter approach.) Due to the processing gain, preambledetection may occur under conditions of low C/I where demodulation willnot occur. Accordingly, there may be two types of preamble detection:with and without demodulation. This allows for preamble detection at anAP from many STAs and other APs that are beyond the communication range(e.g., too great a path loss) as well as in the presence ofinterference. The measured signal strength from this preamble detectionof frames received from other STAs or APs at all the beams/antennaelements of a given AP may be denoted as the spatial signature vector(or in short: spatial signature) of these other devices. The aggregationof this information for all the STAs and APs is the channel gain matrix,which may also be denoted as a spatial signature matrix.

A preamble may be detected without demodulation for a number of reasons,such as Forward Error Correction (FEC) failure, insufficient C/I to evenattempt demodulation, or preemption by demodulation of another frame. Adetection of frame preamble without demodulation provides an AP withinformation that includes the receiving antenna element, the preamblestart time, the signal strength, and the C/I. An educated guess may thenbe made as to the source STA of this preamble so that its spatialsignature may be updated. For example, the start time of a preambledetection may be matched to the start time of a valid frame demodulationto determine the source. Because the start time of valid framedemodulations can only be known after the complete packet is received, acertain amount of latency is required. As a result, preamble detectionsmay be queued in the order of arrival until they can be matched tocorresponding frame demodulations.

If the preamble detection occurs with demodulation, the availableinformation includes the source BSS identification number, STAidentification number or AP identification number, the receiving antennaelement, preamble start time, signal strength, and C/I. If the sourceSTA or AP is part of the relevant BSS, then the corresponding spatialsignature may be updated as a “high” confidence update. Updates to thespatial signature vector based on preamble detections withoutdemodulation, i.e., matching the start time of a preamble detection withthe start time of a valid frame demodulation, are “low” confidenceupdates since there is a possibility the match is in error.

Although preambles are convenient for determining the spatial signaturematrix, cross-correlation of entire packets may also be used to buildthis information. Cross-correlation may be used to perform entire packetdetection on any packets whose content is substantially known, e.g.,acknowledgement (ACK) and Clear-to-Send (CTS). Both MAC frames are 14octets long. The power management bit in the frame control field is theonly bit unknown to the receiver (the value of this bit also determinesthe FCS). Coordinated scheduling by the central controller can takeadvantage of cross-correlation packet detection and allow othercommunications which would interfere with frame demodulation during thetime an AP performs cross-correlation packet detection.

As explained above, there may be two types of updates to a spatialsignature: high or low confidence. Scheduling of frames on the downlinkfrom APs to STAs may or may not account for such confidence values. Inone embodiment, an assumption may be made that the error rate on lowconfidence measures is sufficiently low such that low confidence valuesmay be used the same as high confidence values. Each entry in thespatial signature matrix consists of a set of the most recently measuredsignal strengths. The set may comprise the 16 most recent measurements,according to one embodiment. The central controller may construct ahistogram out of the measurements and select the mode of thedistribution. Ties may be broken by determining which one of the tiedbins contains the more recent measurement.

In addition to the spatial signature matrix, central controller 20 mayneed to consider additional data before a proper frame scheduling may beeffected. For example, scheduling of frames on the downlink from APs toSTAs may also account for the probability that a STA transmitting an ACKon the uplink will interfere with a downlink frame going to another STA.This probability that one STA transmission will interfere with anotherSTA reception is governed by the STA-to-STA channel gains. Assuming thatterminals are not changed, a direct estimation of these gains may beimpossible. However, the above-described ability to estimate theAP-to-STA channel gain provides a technique to statistically estimatethe STA-to-STA interference probability. If the path loss from each STAto relatively many APs are known, the STA-to-STA interferenceprobability can be estimated with sufficient confidence. For example,central controller 20 may examine the cross-correlation of spatialsignatures vectors for two STAs. The expectation is that a pair of STAswith more highly correlated spatial signatures will likely have lesspath loss between them (a high STA-to-STA interference probability).

Whether the transmission of one station will interfere with thereception of another station depends both on the strength of theinterference (estimated from the spatial signature) and the receivingsignal strength. The strength of the interference is estimated from thecross-correlation of the spatial signatures. Highly correlatedsignatures indicate RF proximity and hence higher probability ofinterference, while low correlation indicates lower probability ofinterference. In the simplest model, the spatial signaturecross-correlation is compared to a threshold to give a binary (0,1)probability of interference. To compensate for differences in expectedreceived signal strength at the station (due to the varying distances ofstations to access points), the threshold is maintained on a per stationbasis. To account for uncertainty in the estimates and lack ofmeasurements needed to set the threshold value, the threshold isadaptive based on transmission success history. For stations which theaccess point re-transmits at a low rate, the sensitivity tocross-correlation related error is decreased by increasing thecross-correlation threshold. For those stations that the access pointre-transmits to at a higher rate, decreasing the threshold increases thesensitivity to cross-correlation.

Referring now to FIG. 4, cells 10 (or coverage areas) for three APs(AP#1, AP#2, and AP#3) are illustrated. A first STA (not illustrated)that has low path loss to AP#1 and AP#2 and a second STA (also notillustrated) that has similar low path losses to AP#2 and AP#3 will havelow probability of interfering with one another. The location of suchSTAs is denoted as the low STA-to-STA correlation areas in contrast tothe high STA-to-STA correlation areas where STAs have a relatively highprobability of interfering with one another. To have low STA-to-STAprobability, the sets of low path losses between the STA and the APsshould be as orthogonal as possible. The likelihood of such a result isincreased when sufficient number of channels between each STA to APs canbe estimated (as facilitated by the proposed cross-correlation methodand more STAs are served). The ability to estimate STA-to-STA path losscan also be increased if each AP uses a multi-beam antenna, which is theequivalent of increasing the number of APs in the network.

According to another embodiment, estimating STA-to-STA channel gainestimation is based upon the success of previous transmissions. Here,central controller 20 allows an asynchronous transmission. If anexpected ACK is not received, then central controller 20 may surmisethat there was interference from another station. It may then trackthese surmised interferences as a probability of interfering. Asanticipated packets are missed, central controller 20 may increase theprobability of interferences. As they are received, controller 20 maydecrease these probabilities. Regardless of how the STA-to-STA gain isestimated, the goal is to simply decrease the probability ofinterference from asynchronous communication. As is know to thoseskilled in the art, numerous other methods may be utilized to reducethis probability of interference.

Central controller 20 can only determine the spatial signature matrix(and from this information infer the STA-to-STA interferenceprobability) if all STAs and APs within its network are communicatingregularly. The spatial signature of a station is not static. It dependson both the station itself and the environment.

To update the spatial signatures, the central controller may perform aprocess denoted as probing to discover and update the spatialsignatures. For example, central controller 20 may implement a statemachine 200 illustrated in FIG. 12 for each STA. Table 1 below describesthe states for state machine 200:

TABLE 1 State Description EXPIRED Current spatial signature is invalid.Transition to the EXPIRED AND PROBING state creates a Probing Requestevent to start the probing process for this STA. EXPIRED AND Currentspatial signature is invalid. Probing to update current spatial PROBINGsignature is underway. EXPIRED AND Current spatial signature is invalid.Probing failed to discover current PROBING FAILED spatial signature.VALID Current spatial signature is valid. Transition to the VALID ANDPROBING state creates a ProbingRequest event to start the probingprocess for this STA. VALID AND Current spatial signature is valid.Probing to update current spatial PROBING signature is underway.

The transition events are summarized in Table 2 below:

TABLE 2 Transition Events Description Signature Request Generated whenspatial signature is needed and current signature is not valid. ProbingSuccess Generated when probing has been successful and spatial signaturehas been updated. Probing Failure Generated when probing has failed.Expire Timer Generated when current spatial signature has expires.(Default: 5 sec) RevalidateTimer & Generated in order to prevent spatialsignature from expiring. This is a ActiveSTAFlag combination of a timerexpiration (Default: 4 sec) and an indication that this station isactive. LinkFailure Generated when MAC determines link is down due tolack of responses from STA.

State machine 200 does not include any events that indicate a detectionthat the signature may have changed. Such an event may be added inalternate embodiments of this state machine. In addition, state machine200 does not accommodate any resetting of the timers in the event thatcomponents of spatial signatures are updated regularly through use ofthe communication link. In alternate embodiments, the timers may bereset accordingly. Finally, alternate embodiments may be configured tonever have a timer expiration. Thus, in such embodiments, polling wouldonly occur based on a detected link failure.

All probing for a STA is initiated via a ProbingRequest event generatedby state machine 200. This basic probing process involves selecting anAP, transmitting a packet (such as an RTS) from the selected AP to theSTA that is to be polled, and then listening on all APs for the response(for example, an ACK). If there is no response, then another AP may beselected and the process repeated. Once there is a response, then thespatial signature is measured and the probing is completed for this STA.If there is no response from any of the access points, then the probinghas temporarily failed. This could occur because the STA has reallydisappeared or because of interference or other temporary changes in theenvironment. Thus, it is important to repeat the process with randombackoff. The following is an algorithm for probing during a contentionfree period (CFP).

-   1.0 Construct ordered list of APs    -   1.1 Using the current spatial signature, determine APs that        could last communicate with an STA to be polled. Order these APs        by decreasing value of the greatest magnitude response of all        beams at each AP.    -   1.2 Order remaining APs randomly at end of list.-   2.0 Search for STA from APs in ordered list    -   2.1 Remove first AP from ordered list.    -   2.2 Send RTS to STA from AP (in an omni-directional fashion        using all available antenna beams). All other APs listen (also        in an omni-directional fashion) for the RTS and update their        AP-AP spatial signatures accordingly. Duration value of the RTS        is set to the time required to send one CTS frame plus one SIFS        interval.    -   2.3 All APs listen (omni-directionally) for CTS.    -   2.4 If no CTS is received at any AP, return to step 2.1.    -   2.5 If a CTS is received at one or more APs, update the spatial        signature accordingly. Then create a ProbingSuccess event and        proceed to step 4.0.-   3.0 Reschedule probing    -   3.1 Use a random exponential backoff to reschedule probing        starting at Step 1.0.    -   3.2 If the polled STA has not been discovered after Probing        Timeout seconds, give up and create Probing Failure event.-   4.0 END

It will be appreciated that the foregoing algorithm may be altered suchthat only a subset of the available APs listen for a response based onthe last known spatial signature. As a result, central controller 20may, in parallel, probe a number of STAs with disjoint sets of APs. Suchan approach may be used when there is no indication of any changes inspatial signature. However, if there was an indication of change, thefull probing algorithm may be implemented.

Rather than probe during a contention-free period (CFP), probing mayalso occur during a contention period (CP). This is more difficult sinceSTAs may contend causing interference in the probing process.

The RTS is a 20 byte MPDU and the CTS is a 16 byte MPDU in 802.11 WLANs.For 802.11a, at 6 Mbps, the packet transmit times are 52 and 42microseconds, respectively, for RTS and CTS. For 802.11b, at 5.5 Mbps,the packet transmit times are 222 and 213 microseconds, respectively,for RTS and CTS. A successful probe consists of the sum of SIFS(shortest inter-frame sequence)+RTS+SIFS+CTS, for a total time of 126microseconds for 802.11a and 455 microseconds for 802.11b. It will beappreciated that the total system time spent probing scales linearlywith the number of active stations. On the other hand, the time to finda lost station scales linearly with the number of APs. Thus, the fullprobing algorithm does not scale well. If, however, probing is basedupon partial sets of APs as discussed above, the scaling is improved.

Having determined the spatial signature matrix (or channel gain matrix)and related STA-to-STA interferences, central controller 20 may thenschedule frame transmissions from its APs to its STAs based upon theexpected probability of success for a given transmission. Referring backto FIG. 2, central controller 20 may use its channel gain matrix toperform frame scheduling. Users on a LAN 21 coupled to centralcontroller 20 send frames to central controller 20, which queues theframes. The channel gain matrix contains values for each possibleconnection between APs 25 and 30 and STAs 35 and 40. For example, G2represents the gain between STA 35 and AP 25. Should the frame queuecontain a first frame destined for STA 35 and a second frame destinedfor STA 40, central controller 20 may command AP 25 to send the firstframe and AP 30 to send the second frame simultaneously if G3 issufficiently larger than G5, G4, and G1 (for both transmission and ACK)and if G2 is sufficiently larger than G4, G5, and G1. It will beappreciated that “simultaneous” frame transmission denotes thescheduling an AP frame transmission during the duration of another AP'sframe transmission. Should central controller 20 decide to command thesequential transmission of the first and second frames, it will commandthe transmission timing to be such that one STA transmission will notinterfere with another STA reception based upon the STA-to-STA gain. Thesequential decision would be based upon a given STA not being compatiblewith other STAs for receiving a simultaneous transmission based upon thechannel gain matrix.

If a given STA is repeatedly found unsuitable for simultaneous servicewith other STAs in a WLAN, central controller 20 may assign differentfrequency channels to the “unfitted” given STA. For example, FIG. 6shows an exemplary network coverage plan 100. Each network 110 operateson an assigned frequency channel such that adjacent networks do notshare the same frequency channel. Within each network are multiple APscommunicating with multiple STAs as described with respect to FIG. 2. Acentral controller (not illustrated) controls the frame scheduling andfrequency assignments for each network 110. Should the centralcontroller detect that a given STA has been repeatedly found unsuitablefor simultaneous transmission with other STAs, the central controllercan direct the given STA to a different frequency channel using theapplication layer agent within the STA. The application layer agentcould also set its STA's transmission (TX) power from time to time. Inthis fashion, the frequency channel assignment is used much moreefficiently than in a conventional DCF-based network. Such scalabilityalso lends the present invention to larger networks with increasedcoverage areas.

When different frame lengths are to be simultaneously transmitted on adownlink to multiple STAs in a given network/frequency assignment, thereis a risk that one AP transmitting on downlink may interfere with theACK coming from an STA in response to a shorter frame receipt. Since thecontent of the incoming ACK message is well known to the sending AP, theACK frame may be intercepted using cross-correlation with the knownmessage content at the sending AP. This makes the acknowledgementprocesses much more robust.

The WLAN transmission period can be divided between downlink and uplinkbased on traffic load ratio between them. The WLAN may use the PCFsub-period for downlink and DCF period for uplink. In one embodiment,the initial division can be ½ to ½. The uplink portion may then begradually reduced until its utilization is increased to 100%. Since, inmost cases, the traffic on the uplink is expected to be much lessintensive than on downlink, and since the WLAN has good knowledge on theamount of traffic expected on the downlink, such an uplink/downlinkdivision can provide flexibility and performance.

Referring back to FIG. 2, with respect to uplink transmissions from STAs35 and 40, a conventional DCF or PCF regime may be used. If PCF isimplemented, the multiple antenna elements for each AP provide fordiversity gain and dynamic selection of receiving frames as they do indownlink. Even if conventional DCF would be used on uplink, STAs 35 and40 will have less interference than in conventional WLANs due to theability to reduce their transmission power.

In addition, the possibility of lower STA transmit power reduces theprobability of uplink suppression. Moreover, each STA 35 and 40 may havea higher effective data rate because the diversity gain improves thelink budget. However, to realize diversity gain benefits, the PCF modeshould be implemented to assure an AP's best antenna beam is directed toa STA when it transmits.

To achieve the diversity gain benefits, multiple transceivers per AP maybe used such that each transceiver demodulates the output of a specificbeam. The AP then selects the best beam/antenna element based on theframe's CRC. This arrangement provides for 4-brunch selection diversity,thereby providing a significant performance advantage. An AP receivingmultiple uplink frames from STAs operating should select the strongestreceived frame because central controller 20 will assure that other APsare handling the remaining frames.

Example of a Scheduling Algorithm

The scheduling algorithm implemented by the central controller isdesigned to operate without complete information about the signalstrengths measured at different APs from the various STAs to which itmust transmit. The central controller may schedule packets in its packetqueue as quickly as possible, by attempting to transmit to as manystations as possible at the same time, based on the availableinformation.

The scheduling algorithm may make use of four basic sets of information:

1. AP Interaction Table

2. Station Interaction Table

3. Signal Strength Table

4. Transmission History Table

The AP Interaction Table comprises the AP-to-AP spatial signatureinformation described previously. Relevant information in the table mayinclude signal strength, beam identification, frequency channelidentification, and a time stamp. The Station Interaction Tablecomprises the STA-to-STA interference data described previously. Eachentry may include data indicating whether each pair of stations willlikely interfere with one another or allow adequate communication. Forexample, this table may comprise binary entries (either ones or zeroes),where a 1 indicates stations that may work together without significantinterference and a zero indicates stations that interfere with oneanother at a level unacceptable to the system. Alternatively, the tablemay include numerical values representing probability ofnon-interference between two stations. These entries may also includethe channel identification and a time stamp.

The Signal Strength Table comprises the AP/STA portion of the spatialsignature matrix described previously. Measurements are taken each timea STA transmits in the uplink. After a measurement has been kept for acertain period of time, the measurement is deleted, unless it is theonly measurement for that entry. “Old” measurements may be designed assuch. Each measurement should include signal strength informationmeasured at all the beams, a measurement quality indicator, frequencychannel identification, and a time stamp. The Transmission History Tablekeeps a history of the downlink transmissions for a period of time. Thetable includes all simultaneous transmission at a given channel at agiven time. The table may include the start of time of the transmittedpacket, the AP, beam identification, channel identification, the STA,and whether an ACK was received.

Given the above sets of information, a frame scheduling algorithmaccording to the invention may proceed as follows. Note that thefollowing is an illustration and not limiting. The central controllerforms a packet queue based upon the received packet order and theirdestinations. A separate queue may be formed for each frequency channel.As a first step, the central controller may examine the oldest packet inthe queue and assign a first AP to transmit it by looking up in theSignal Strength Table the AP with the largest signal strengthcorresponding to the destination station. The central controller maythen check the Station Interaction Table for a list of the stationswhich are non-interfering with the destination station and check the APInteraction Table for a list of the APs which can operate in anon-interfering manner with the first selected AP. Based upon theseexaminations, the central controller may pick the non-interferingstation which is highest on the Packet Queue, which lists the packetsand their respective destinations, and choose one of the non-interferingAPs to transmit it that will assure the largest received signalstrength. If no non-interfering station can be found, the centralprocessor terminates the search.

Assuming the search has been successful, there are now two stationsassigned to two APs. By examining the Signal Strength Table, the centralprocessor may determine the expected C/I for each of the 2 stations. Ifeach C/I is adequate (based on system requirements), the centralprocessor may proceed by checking the Transmission History Table todetermine whether there has been a recent simultaneous transmission tothese stations, and whether it succeeded or failed. If there was afailure, the central processor may choose a different AP and try again.

Once a workable second station/AP combination has been found, the aboveprocess may be repeated to identify a third compatible station/APcombination, and so on. The search process will terminate when no morecompatible combinations can be found or when a maximum search time hasbeen reached. At this point, the identified APs may simultaneouslytransmit to their selected STAs.

The procedure described above is a sequential search which does notattempt to look at all the possibilities. In other words, a particularstation is chosen as a starting point, then the first compatible stationadded, and so on. Accordingly, the scheduling algorithm is essentiallymoving along a single branch of a complex tree. This procedure isrelatively computationally efficient, but has a relatively highprobability of terminating with a small number of stations chosen forsimultaneous transmission. To increase the average number ofsimultaneous transmissions, the algorithm may be modified to explore themultiple branches of the tree of possibilities. This can be accomplishedby launching multiple instances of the search process at each level ofthe search. Consider for example the first step of the search whereinthe strongest AP is assigned to the station on top of the queue.Instead, the algorithm could consider, for example, the three strongestAPs and continue exploring the three branches thus created. Similarly,instead of choosing the first compatible second station, the algorithmcould choose several compatible stations, and continue the search fromthere. These multiple searches lend themselves very well to parallelcomputation. Thus, the basic algorithm described earlier can bereplicated on multiple processors to save computation time.

To further speed up the assignments, the algorithm may use theinformation collected in these tables to pre-compute various station/APassignments which are known to be compatible. For example, stationsreceiving frames from groups of APs which have a large path loss inbetween them are very likely to be non-interfering. These groups of APsmay be identified as “safe” groups. Then, if the algorithm is schedulingframes to multiple stations, each related to a different “safe” APgroup, it may immediately assign them to a member of each group. Inanother embodiment of the algorithm, the Transmission History Table maybe examined to prepare lists of station/AP combinations that have beenworking successfully multiple times in the recent past. Whenever such acombination of stations comes up, the algorithm could schedule frametransmissions immediately. Once a particular combination fails, it isremoved from the list.

More generally, a separate table of special cases may be prepared inwhich an immediate assignment can be made. In parallel with the generalsearch process described earlier, the central controller may perform aseparate search process looking for these special cases. Whenever such aspecial case is detected, a faster decision can be made.

FIG. 5 a shows simulation results for a WLAN with 9 APs and 100 randomlydistributed STAs, with a 25% probability of transmission pathobstructions. The vertical line shows the cross-correlation thresholdselected and the horizontal line is the interference thresholdrepresented by 10 dB channel/interference (C/I) conditions at each STA.At a cross-correlation of 0.9, about 50% of the STAs can be selected forsimultaneous service since the STA-to-STA interference does not reach 10dB C/I with less than 0.3% error probability. The number ofsimultaneously serviceable STAs can be increased by selecting a highercross correlation threshold. FIG. 5 b shows simulation results for thesame WLAN in which the spatial signature cross-correlation is increasedto 0.95. The percentage of simultaneously serviceable STAs increases toabout 75%, with an error probability of about 3%.

The cross correlation threshold can be used as a starting point foradaptive threshold, based on success/failure history. A threshold can bedefined for each STA and adjusted up or down depending on thesuccess/failure of frames delivery to that STA. This way an aprioridecision on threshold value is not important. Furthermore, an adaptivethreshold reduces the need for a highly accurate spatial signature or aspatial signature having a large number of elements. Consequently, muchlower cross C/I and signal level measurement dynamic range is required,making most RSSI exiting AP mechanisms suitable for the job.

Examples of AP Embodiments

Turning now to FIG. 7, an exemplary block diagram for an AP is shown. Anantenna array 120 in conjunction with a beam forming network 125 such asa Butler matrix forms multiple antenna beams such as antenna beams 130whose gains are illustrated in FIG. 8. The resulting antenna beamsshould be orthogonal to one another. Antenna array 120 may be designedfor omni-directional or sector coverage depending upon the application.A test receiver 135 scans the antenna array ports through switch 140 todetermine the STA responses or SNR at each antenna beam. These responsesare used by the central controller to schedule downlink frametransmissions. I and Q samples from an analog-to-digital converter 150are received by a local AP controller 160 which may perform fastcorrelation functions using a correlator 165. The fast correlation maybe performed using a matched filter on either the preamble or, in thecase of an ACK, CTS or related message, the whole frame. An 802.11chipset 155 performs the required modulation/demodulation and otherfunctions such as MAC.

An alternative embodiment for an AP is shown in FIG. 9. Here, the MACelement of the 802.11 chipset has been moved to the central controller,whereby this embodiment may be denoted as a “simple AP.” Such anembodiment makes coordination between APs easier. Because the connectionbetween a simple AP and its central controller may be either analog ordigital, this connection may carry the modulator output from the MACwithin the central controller and the I/Q outputs from a transceiver170.

A block diagram for a central controller 190 for a network in which eachAP performs its MAC function (as in FIG. 7) is shown in FIG. 10. In thisembodiment, the MAC elements are part of the APs, but the APtransmissions must still be coordinated. Central controller 190 willdetermine the transmission attributes, but because the MAC at each APexecutes the actual transmission, a synch generator 191 should beincluded. Each AP will use synchronize transmissions according to asynch signal from synch generator 191.

In FIG. 11, a central controller 180 having a centralized MAC for simpleAPs 175 (FIG. 9) is illustrated. To form the channel gain matrix,central controller 180 may gather analog beam responses from simple APs175 or these beam responses may be measured within simple APs 175 andreported to central controller 180. Central controller 180 uses itschannel gain matrix to assign frames to be transmitted to a specificsimple AP 175 following its scheduling algorithm as implemented by aprocessor 200. The frames to be transmitted may be stored in a TX buffer190 and received frames from STAs may be stored in an RX buffer 195.Processor 200 may decide for each frame in TX buffer 198 what attributeswill be used. For example, theses attributes may include the STAdestination, STA beam number or array number (should multiple arrays beimplemented), time of transmission, TX power, and phase.

It will be appreciated that the frame scheduling algorithms disclosedherein may be subject to many modifications and still schedule frametransmissions based upon a predetermined likelihood of transmissionsuccess. Accordingly, although the invention has been described withrespect to particular embodiments, this description is only an exampleof the invention's application and should not be taken as a limitation.Consequently, the scope of the invention is set forth in the followingclaims.

What is claimed is:
 1. An access point to wirelessly communicate with astation, said access point comprising: an array of antennas, whereineach of the antennas comprises a port; a beam forming network to formmultiple beams from the array of antennas; a switch connected to thebeam forming network; a test receiver to scan the ports of the array ofantennas through the switch to determine at least one of (1) responsesfrom the station and (2) signal-to-noise ratios (SNRs) of the station;an analog transceiver coupled to the switch; wherein the access point isto communicate the at least one of (1) responses from the station and(2) SNRs of the station to a central controller for use by the centralcontroller to schedule downlink frame transmissions for the accesspoint, and wherein the access point is to communicate with the centralcontroller using the analog transceiver.
 2. The access point accordingto claim 1, wherein the multiple beams formed from the array of antennasare orthogonal to each other.
 3. The access point according to claim 1,wherein the array of antennas are designed for one of omni-directionaland sector coverage.
 4. The access point according to claim 1, whereinthe access point is to receive frame transmissions from the centralcontroller and to communicate the frame transmissions to the station. 5.The access point according to claim 1, further comprising: an 802.11chipset to modulate and demodulate the at least one of (1) responsesfrom the station and (2) SNRs of the station.
 6. The access pointaccording to claim 1, further comprising: an analog-to-digitalconverter; a correlator; and a controller to receive samples convertedby the analog-to-digital converter and to perform fast correlationfunctions using the correlator.
 7. The access point according to claim1, wherein the access point does not include a 802.11 chipset.
 8. Theaccess point of claim 1, further comprising: a plurality of linetransceivers coupled to the analog transceiver, wherein the access pointis to communicate with the central controller through the plurality ofline transceivers.
 9. A method of implementing an access point towirelessly communicate with a station, said method comprising:determining, in the access point, at least one of: (1) responses fromthe station and (2) signal-to-noise ratios (SNRs) of the station;communicating, to a central controller via an analog transceiver of theaccess point, the at least one of: (1) responses from the station and(2) SNRs of the station for the central controller to schedule downlinkframe transmissions for the access point; receiving the scheduleddownlink frame transmissions from the central controller; andcommunicating the frame transmissions to the station.
 10. The methodaccording to claim 9, wherein the access point comprises an array ofantennas and a beam forming network to form multiple beams from thearray of antennas, and wherein the access point is to receive uplinkframes from the station through the multiple beams, said method furthercomprising: selecting one of the multiple beams to communicate with thestation based upon characteristics of the uplink frames received throughthe array of antennas.
 11. The method according to claim 9, whereindetermining, in the access point, at least one of responses from thestation and SNRs of the station further comprises determining at leastone of channel gains between the access point and the station,probability of station to station interference, access point transceiverto station path loss, and station to station path loss information. 12.The method according to claim 9, further comprising: polling, by theaccess point, the station when an uplink frame transmission from thestation has not occurred for a specified period of time.
 13. The methodof claim 9, wherein a plurality of line transceivers are coupled to theanalog transceiver of the access point, and wherein the access pointcommunicates with the central controller through the plurality of linetransceivers.